Some users pointed us a problem with Black Ink since the latest Windows Creator Update like this post and this oneFrom what we understand there is a problem with the Window compositing when there's some transparency.We will work to fix this issue for the next update but for now here the instruction to workaround:

Open the Config.xml file with a text editor (On Windows 7/Vista):C:\Users\MyUserName\AppData\Roaming\Bleank\BlackInk\Config.xml